agent: Remove selections as context once message is sent (#29030)

Release Notes:

- N/A

crates/agent/src/message_editor.rs

@@ -2,7 +2,7 @@ use std::collections::BTreeMap;
 use std::sync::Arc;
 
 use crate::assistant_model_selector::ModelType;
-use crate::context::format_context_as_string;
+use crate::context::{AssistantContext, format_context_as_string};
 use crate::tool_compatibility::{IncompatibleToolsState, IncompatibleToolsTooltip};
 use buffer_diff::BufferDiff;
 use collections::HashSet;
@@ -293,6 +293,21 @@ impl MessageEditor {
                 })
                 .log_err();
 
+            context_store
+                .update(cx, |context_store, cx| {
+                    let excerpt_ids = context_store
+                        .context()
+                        .iter()
+                        .filter(|ctx| matches!(ctx, AssistantContext::Excerpt(_)))
+                        .map(|ctx| ctx.id())
+                        .collect::<Vec<_>>();
+
+                    for id in excerpt_ids {
+                        context_store.remove_context(id, cx);
+                    }
+                })
+                .log_err();
+
             if let Some(wait_for_summaries) = context_store
                 .update(cx, |context_store, cx| context_store.wait_for_summaries(cx))
                 .log_err()